A turbofan engine has a propulsive fan and compressor driven by a series of turbine sections. The turbine sections are connected to the fan and compressor by a series of mainline shafts extending in the axial direction of the engine.
In the event of a mainline shaft failure the decoupled turbine has a tendency to overspeed, potentially leading to failure of a turbine disc. It is advantageous to allow the rear portion of the failed shaft to move rearwards leading to tangling in the turbine, thereby reducing overspeed.
In some engine designs, in the event on a mainline shaft failure rearwards movement of the rear section of the shaft can be limited due to a clash in a roller bearing. In such designs, the roller bearing race can be connected to the mainline shaft via a stub shaft.